A well-known type of semiconductor integrated circuit in which memory cells are integrated on a large scale (hereinbelow termed the "semiconductor memory") is the so-called RAM. The RAM (random access memory) is a device capable of storing information temporarily and reading it out when required. This type of memory is also called a "read/write memory".
Typically, a RAM includes memory cells which store information, an address circuit which externally selects a specified memory cell, and a timing circuit which controls the reading and writing of information.
In a RAM, a plurality of memory cells are arranged in the shape of a matrix. The operation of selecting a desired memory cell from among the plurality of memory cells is performed by selecting an intersection point in the matrix. The access time is therefore constant irrespective of the position (addresses) of the selected memory cells within the matrix.
RAMs are broadly classified into two sorts; bipolar RAMs and MOSRAMs.
The bipolar RAM has the following merits:
(1) As compared with the MOSRAM, it operates faster.
(2) The operation of the memory cell is of the static type, and the controls of timings, etc. are simple.
On the other hand, the bipolar RAM has the following demerits:
(3) As compared with the MOSRAM, it exhibits a higher power consumption (especially when it does not operate).
(4) As compared with the MOSRAM, it requires a more complicated manufacturing process and is more difficult to attain a high density of integration.
Bipolar RAMs are presently generally classified into the two types of the TTL type and the ECL type, depending upon differences in input/output levels. The access time (reading time) of the bipolar RAM of TTL interface falls within a range of 30-60 (nsec.), while the access time of the bipolar RAM of ECL interface falls within a range of 4-35 (nsec.).
Accordingly, bipolar RAMs are applied to various memory systems where high speed operations are required.
Meanwhile, when compared with the bipolar RAM, the MOSRAM is simpler in structure and in the manufacturing process. It is also more advantageous in terms of power consumption, storage density and price. Therefore, it is used in fields which do not require high speed operations.
MOSRAMs are classified into the dynamic type and the static type.
The dynamic type MOSRAM has its memory cell composed of a comparatively small number of transistors, namely, 1-3 transistors per bit (1-3 transistors/bit). With an identical chip area, therefore, the bit density becomes higher than that of the static type MOSRAM to be described later.
In the dynamic MOSRAM, information is stored as charges in a capacitance within the memory cell. Since the charges stored in the capacitance are discharged due to a leakage current, etc., the information of the memory cell needs to be read out within a predetermined period of time and to be rewritten again (i.e., refreshed).
On the other hand, in the static MOSRAM, a flip-flop circuit which is usually composed of 6 elements is used as the memory cell. For this reason, the refresh which is required in the dynamic MOSRAM is not necessary.
The access time of the dynamic MOSRAM falls within a range of 100-300 (nsec.), while the access time of the static MOSRAM falls within a range of 30-200 (nsec.). Thus, it can be seen that the access time of the MOSRAM is a larger value when compared with that of the bipolar RAM.
Meanwhile, owing to improvements in photolithographic technology, reduction in the element dimensions of MISFETs within a semiconductor integrated circuit has been promoted. In IEEE Journal of Solid-State Circuit, Vol. SC-17, No. 5, pp. 793-797, issued in Oct. 1982, there is contained a static MOSRAM of 64 kbits which employs wafer processing techniques based on design rules of 2 (.mu.m) and which exhibits an access time of 65 (nsec.), an operating power consumption of 200 (mW) and a stand-by power consumption of 10 (.mu.W).
Meanwhile, as an example of the bipolar RAM of the ECL type, an ECL type bipolar RAM of 4 kbits which exhibits an access time of 15 (nsec.) and a power consumption of 800 (mW) is manufactured and sold by Hitachi, Ltd. under the product name "HM100474-15".
As explained above, there has been a definite technical trend to enlarge the storage capacity of semiconductor memories which has taken place in the increments of 1 kbit, 4 kbits, 16 kbits, 64 kbits, 256 kbits, 1 Mbit, . . . , quite independently of the features of the bipolar RAM of high speed and high power consumption and the features of the MOSRAM of low speed and low power consumption.
At the present time, when the power consumption of the semiconductor memory and the present-day photolithographic techniques determining the element dimensions of bipolar transistors are taken into consideration, the storage capacity of the bipolar RAM will be limited to 16 kbits.
Meanwhile, with the enlargement of the storage capacity of the semiconductor memory (particularly, at and above 64 kbits), the area of a semiconductor chip increases, and the signal line of the address circuit of the RAM is arranged over a long distance on the semiconductor chip of large area. When the length of the signal line of the address circuit lengthens, naturally the stray capacitance of the signal line increases, and also the equivalent distributed resistance of the signal line increases. When, for the purpose of microminiaturization, the wiring width of the signal line of the address circuit is established as 2 (.mu.m) or less by improving photolithography, the equivalent distributed resistance of the signal line increases more. In addition, since the fan-out of each circuit enlarges with the increase of the storage capacity, a load capacitance attributed to the gate capacitance of a MOSFET at the succeeding stage becomes high. Accordingly, in the 64-kbit MOSRAM which employs the photolithography of 2 (.mu.m) and whose address circuit is entirely constructed of CMOSFETs, the access time of addresses will be limited to 30 (nsec.).
The present invention has been made by the inventors in developing a semiconductor memory which has an access time equivalent to that of an ECL type bipolar RAM and a power consumption equivalent to that of a static MOSRAM.